Severe Acute Respiratory Syndrome (Sars) is thought to be a corona virus, a family of viruses well known as a cause of the common cold, which has mutated to become more dangerous to humans.

The virus first appeared in China's Guangdong province in November, and has since spread around the world.

Airline passengers have taken the virus to North America - 14 people have died in Canada - and Europe.

Many of the deaths have been among the elderly, but victims have also included an otherwise healthy 37-year-old man and two people in their mid-50s.

It is likely that infection takes place through body fluids, such as droplets produced in sneezes and coughs, but the World Health Organisation (WHO) says transmission may also occur through touching objects such as lift buttons.

It is unclear how much protection face masks offer against an airborne virus.

Symptoms include a sudden fever, a temperature of more than 38C, followed by coughing, shortness of breath and difficulty breathing. The incubation period is from two to seven days, although it can be as long as ten days.

Sufferers are being treated with anti-viral drugs, although it is not clear how much effect these are having. The mortality rate is around four per cent, higher than many other respiratory infections, although considerably lower than diseases which are considered to be virulent, such as ebola. The majority of sufferers recover within seven days.

The WHO has issued a global alert, asking doctors to look out for cases, particularly among patients who have recently returned from abroad. Airlines have been asked to look out for passengers who appear unwell, and have been advised to disinfect aircraft.

The Department of Health has advised people not to travel to Hong Kong or the Guangdong province, and travellers returning from an infected area should seek medical advice if they experience any of the symptoms.

The ability of the Sars virus to mutate means that any vaccine is likely to be effective for only a short time. Experts believe that once the virus has become established, it will be virtually impossible to eliminate.